After a lot of creative searching I discover it is possible to set up a Mac to permit Oxford spelling (organize vs organise).
Oxford Spelling
Many words formed by adding -ize to a noun or adjective have two correct spellings in British English, one ending -ize and the other -ise, for example organize and organise, realize and realise.
The -ize ending is more directly related to the etymology (from the Greek as opposed to indirectly via French), corresponds to the pronunciation, and is used by Shakespeare, Tolkein, the United Nations, ISO, and other international institutions. This is, or was, the preferred spelling for the Oxford University Press house style, and is generally known outside Oxford as Oxford spelling.
Given a choice, I prefer the Oxford spelling myself.
The Tyranny of American British English
The (American) companies that supply spelling dictionaries assume wrongly that -ize is incorrect in British English. This makes following Oxford spelling a continual fight against the spell-checker.
As a result, many organizations whose house style specified Oxford spelling in the past have given up and switched to specifying -ise for the sake of a quiet life.
So we need a dictionary for English with Oxford Spelling—what OUP might prefer to call World English.
Available Dictionaries
There are rumours that spelling dictionaries for Oxford spelling exist,
but if they do they are not at all easy to find. Links to dictionaries (like
in this exchange) are invariably to non-existent servers, or wiki pages
that have been replaced with a link to an extensions marketplace that doesn’t
have anything suitable. You can google for the language
tags en-GB-oxendict (from the IANA registry) or its grandfathered
equivalent en-GB-oed, but you mostly find plaintive bug reports.
What I eventually found this was is a dictionary en-GB-oed version R 1.19, edited by
David Bartlett in 2005, that can be downloaded from an attachment on a
long-forgotten OpenOffice bug. But is installing dictionary unmaintained in 14 years
going to be more trouble than it’s worth?
But then a breakthrough.
I was writing a version of this entry which discussed the spell-checking software.
There are several software libraries for spell-checking, with confusing
relationships between them. Hunspell is used by LibreOffice, OpenOffice,
web browsers like Chrome and Safari, and macOS.
Chasing links starting from its GitHub page I found a collection of
dictionaries that includes an en_GB dictionary updated (by Marco A.G. Pinto) as recently as
2018. Eyeballing its .aff file suggests it
permits -ize as well as -ise. Great!
Installing Spelling on macOS X
I can install the dictionary by copying the .dic and .aff files in to
~/Library/Spelling and restarting my Mac. Once I had restarted I could
visit System Preferences → Keyboard → Text → Spelling → Set Up and replace
British English with the dictionary I had added.
This seems to work: I can type organize centre colour in to a text editor and all three words are accepted as correctly spelled.
This solution is unsatisfactory in that it took me half a day of googling and more background technical knowledge that the average writer should be expected to need to implement this solution. I would also have preferred to be able to specify a more strict en-GB-oxendict spelling that would reject organise in favour of organize.